Apparatus for classroom physics experiments

ABSTRACT

Apparatus for demonstrating Newton&#39;&#39;s third law of motion comprises first and second substantially spherical masses. Structure is provided for moving first mass along a path of travel that includes substantially horizontal portion. Support structure is provided for locating second mass in alignment with horizontal portion of path of travel of first mass. Support structure includes vertically disposed support pin with flat support surface having area substantially less than diametral cross sectional area of second mass. Sleeve snugly surrounding support pin is mounted for sliding movement relative to pin, and weight is spaced from and directly above flat support surface of support pin for engaging top of second mass when second mass is positioned upon support surface of pin. Sleeve has upper position where upper surface thereof and support surface of pin are close to one another for locating second mass on support surface. Sleeve has lower position spaced from support surface of pin so that second mass is supported by pin between support surface thereof and weight when sleeve is moved to lower position after assisting in locating second mass on support surface.

United States Patent Chambers 51 Mar. 21, 1972 [54] APPARATUS FOR CLASSROOM PHYSICS EXPERIMENTS [72] Inventor: Robert F. Chambers, 504 Beverly Road,

Newark, Del. 19711 22 Filed: Aug. 13, 1970 [21] Appl.No.: 63,466

[52] US. Cl. ..35/l9 R, 273/120 R [51] Int. Cl. ..G09b 23/10 [58] Field ofSearch ..35/l9 R; 273/120R OTHER PUBLICATIONS Welch Scientific Co. Scientific Apparatus and Supplies Catalog rec d Oct. 1965, p. 68 only.

Primary Examiner-Harland S. Skogquist Attorney-Connolly and Hutz [57] ABSTRACT Apparatus for demonstrating Newton's third law of motion comprises first and second substantially spherical masses. Structure is provided for moving first mass along a path of travel that includes substantially horizontal portion. Support structure is provided for locating second mass in alignment with horizontal portion of path of travel of first mass. Support structure includes vertically disposed support pin with flat support surface having area substantially less than diametral cross sectional area of second mass. Sleeve snugly surrounding support pin is mounted for sliding movement relative to pin, and weight is spaced from and directly above flat support surface of support pin for engaging top of second mass when second mass is positioned upon support surface of pin. Sleeve has upper position where upper surface thereof and support surface of pin are close to one another for locating second mass on support surface. Sleeve has lower position spaced from support surface of pin so that second mass is supported by pin between support surface thereof and weight when sleeve is moved to lower position after assisting in locating second mass on support surface.

9 Claims, 5 Drawing Figures APPARATUS FOR CLASSROOM PHYSICS EXPERIMENTS BACKGROUND OF THE INVENTION The present invention relates to apparatus for classroom physics experiments, and more particularly to apparatus for demonstrating Newtons third law of motion.

Prior to the present invention, numerous arrangements have been proposed for demonstrating phenomena associated with the classroom instruction of physics. The primary considera tion in designing such apparatus is to produce results which consistently demonstrate the particular law being studied. When an experiment is conducted and the deviation from the theoretical result is small the students are favorably impressed with the experiment. On the other hand, when the percentage of error is significantly high the students are not favorably impressed with the outcome whereby the experiment fails to achieve its primary purpose. Additionally, it is important that the apparatus be easy to manage and not complicated to the extent that the mechanics of conducting the experiment are burdensome to the student.

For the most part, apparatus for demonstrating Newton's third law of motion comprises a pair of masses, one moving and one stationary, that collide with one another. Measurements are made which are representative of the relative horizontal momentum of each of the masses after the moving mass collides with the stationary mass. These values are compared with the relative horizontal momentum of the moving mass when a run is made without colliding with the stationary mass. In the past, the stationary mass was supported at a position in the path of travel of the moving mess by many different devices. One such device utilizes a concave seat to support the stationary mass. When the moving mass collides with the stationary mass so supported, the stationary mass must first ride up out of the concave seat and this initial movement adversely affects the experimental data obtained when this type of mass support is utilized. With this arrangement error in the range of to percent is quite common.

SUMMARY OF THE INVENTION Accordingly, the primary object of the present invention is to provide an apparatus for conductingclassroom physics experiments wherein the data obtained by using the apparatus is consistently accurate and wherein the percentage of error or deviation from the theoretical result is negligible.

Another object of the present invention is to provide an apparatus for demonstrating Newton's third law of motion which is easy and simple to operate, andfrom which error free" data is consistently obtained.

In accordance with the present invention an apparatus is provided for demonstrating Newtons third law of motion comprising a first substantially spherical mass and structure for moving the first mass along a path of travel that includes a substantially horizontal portion. A second substantially spheri cal mass is also provided as well as supporting structure for the second mass connected for shifting movement into and out of alignment with the substantially horizontal portion of the path of travel of the first mass. The supporting structure includes a vertically disposed support pin with a flat support surface having an area substantially less than the diametral cross sectional area of the second mass. A sleeve snugly surrounding the support pin is mounted for sliding movement relative to the pin, and a weight is spaced from and directly above the flat support surface of the support pin for engaging the top of the second mass when the second mass is positioned on the support surface of the pin. The sleeve has an upper position where the upper surface thereof and the support surface of the pin are close to one another for locating the second mass on the support surface. Also, the sleeve has a lower position in which it is spaced from the support surface of the pin so that the second mass is supported by the pin between the support surface thereof and the weight after the sleeve assists in positioning the second mass on the pin.

Preferably, the area of the flat support surface of the pin is quite small relative to the diametrical cross sectional area of the second mass. The diametral cross sectional area of the second mass is at least one hundred times greater than the area of the flat support surface of the pin.

The weight of the support structure for the second mass may be hingedly connected to an upright portion on the supporting structure for movement toward and away from the support pin. An adjustable connection is provided between the upright portion and the supporting structure so that the weight may be adjusted in the horizontal and vertical directions. Also, the structure for moving the first mass can include a track having a downwardly inclined first portion with a substantially horizontal second portion at the lower end of the inclined portion. The vertically disposed support pin may be adjustably connected to the support structure for adjustment in the vertical direction so that the support pin may be located so as to position the center of the second mass in the same horizontal plane as the center of the first mass when the second mass is supported by the pin and the first mass is traveling along the horizontal portion of its path of travel. A plumb bob is secured to the lower end of the vertically disposed support pin for locating the second mass in the same horizontal plane as the tip of the plumb bob.

BRIEF DESCRIPTION OF THE DRAWING Novel features and advantages of the present invention in addition to those mentioned above will become apparent to those skilled in the art from a reading of the following detailed description in conjunction with the accompanying drawing wherein:

FIG. 1 is a side elevational view of an apparatus according to the present invention;

FIG. 2 is a top plan view of the apparatus shown in FIG. 1;

FIG. 3 is an enlarged sectional view of the structure for supporting the stationary mass in the path of travel of the moving mass;

FIG. 4 is a top plan view illustrating the relative horizontal velocities of the masses after collision and of the moving mass without colliding with the stationary mass; and

FIG. 5 is a diagram of the relative momentums of the masses after collision showing that the vector sum of these relative momentums equals the relative momentum of the moving mass without colliding with the stationary mass.

DETAILED DESCRIPTION OF THE INVENTION Referring in more particularity to the drawing, FIGS. 1-4 illustrate an apparatus 10 for use in conducting classroom physics experiments. More particularly, the apparatus 10 is quite useful for demonstrating Newton's third law of motion by collecting data to show that momentum is conserved when two masses collide. Specifically, the apparatus 10 comprises a first spherical mass A and a track 12 for moving and guiding the mass A along a desired path of travel. The track 12 has a downwardly inclined first portion 14 with a horizontal second portion 16 at the lower end of the inclined portion. The track is held in position by a mounting 18. A bracket is connected to a pair of C-clamps 22 and the C-clamps are secured to a table 24 or other support surface. As mentioned above, the track 12 functions to move and guide the mass A along its path of travel which includes a horizontal terminal portion.

An electromagnet 26 and appropriate circuitry 28 is provided for holding and releasing the mass A at the same location on the track for each run of the experiment. The circuitry includes a switch 30 and a power source 32. With the switch in its closed position and the mass A is held at the top of the inclined portion 14 of the track 12 and when the switch is opened the mass A is released for travel along the track.

The apparatus 10 also includes a second spherical mass B as well as supporting structure 34 for the mass B connected for shifting movement into and out of alignment with the substantially horizontal portion 16 of the path of travel of the mass A.

As shown best in FIG. 3, the supporting structure 34 is secured to the bracket 20 and includes a vertically disposed support in 36 with a flat support surface 38 having an area which is substantially less than the diametral cross sectional area of mass B. Preferably, the flat support surface 38 is circular and has a diameter of from 1% to 2 millimeters. The diameter of the mass B is from 20 to 30 millimeters. Accordingly, when the diameter of the flat support surface 38 is 2 millimeters and the diameter of the mass B is 20 millimeters, the cross sectional area of mass B is 100 times greater than the area of the flat support surface 38 of the pin 36.

A sleeve 40 snugly surrounds the support pin 36, as shown best in FIG. 3. The sleeve is mounted for sliding movement relative to the pin 36 so that it can be moved upwardly and downwardly relative to the pin. As described below, the sleeve is manipulated to assist in positioning the mass B squarely upon the flat support surface 38 of the pin 36 so that the longitudinal axis of the pin passes through the center of the mass B.

The supporting structure 34 for the second mass B further includes a weight 42 spaced from and directly above the flat support surface 38 of the support pin 36. As shown best in FIG. 3, the weight 42 engages the top of the second mass B when the mass is positioned upon the support surface of the pin. The weight is hingedly connected at 44 to an upright portion 46 of the support structure 34. As is clear, the hinge connection 44 enables the weight 42 to move toward and away from the support pin 36. Also, the hinge connection 44 for the weight 42 is located on the side of the support pin 36 directly opposite the horizontal second portion 16 of the track 12, as shown in FIGS. 1 and 2. An adjustable connection 47 is provided for adjusting the weight 42 in both horizontal and vertical directions. This connection enables the weight to be positioned so as to accommodate masses of varying diameter.

As mentioned above, the supporting structure 34 for the second mass B is connected for shifting movement into and out of alignment with the substantially horizontal portion of the path of travel of the first mass A. In this regard, the supporting structure 34 is pivotally connected to the bracket 20 at 48. A locking assembly 50 is provided for anchoring the supporting structure 34 to the bracket 20. An arcuate slot 52 in the bracket 20 limits the shifting movement of the supporting structure 34, as is clear from FIG. 2 of the drawing.

The apparatus also includes a sheet of paper 54 supported in a horizontal plane by any convenient support surface. For purposes described below, carbon paper 56 or other similar material is positioned over the paper 54. Also, a plumb bob 58 is provided for indicating the center of the mass B in other horizontal planes when that mass is seated upon the flat support surface 38 of the pin 36. The plumb bob is secured to the lower end of the pin 36.

The apparatus 10 of the present invention may be used in the following manner to demonstrate Newtons third law of motion. First, the supporting structure 34 for the second mass B is moved about pivot point 48 out of alignment with the horizontal leg 16 of the track 12. The switch 30 is closed to energize the electromagnet 26. The metal mass A is then positioned at the top of the inclined portion 14 of the track 12 and is held at that position by the electromagnet. The switch 30 is then opened which causes the mass A to roll down the track 12. The momentum of the mass A carries it away from the end of the track 12 until it strikes the carbon paper 56. The carbon paper 56 records the point of impact A on the paper 54, as shown in FIG. 4. The starting point of the mass A is recorded on the paper 54 by a plumb bob (not shown) which is placed at the center of the end of the track 12. Thus, the horizontal starting point A and the horizontal end point A of the mass A is recorded on the paper 54. The distance between points A and A is recorded as the relative horizontal velocity of mass A 1 before impact. Several runs may be taken and averaged to 1 determine an average direction and magnitude for the relative 5 horizontal velocity of mass A before impact.

} Next, the locking assembly 50 is loosened and the support- 5 iing structure 34 is moved toward and into the path of travel of ithe mass A. The locking assembly is then tightened. Mass B is ipositioned upon the flat support surface 38 of the pin 36 in the following manner. First, the sleeve 40 is moved to its upper gposition illustrated in FIG. 3 where the upper surface thereof iand the support surface 38 of the pin are close to one another. Actually, the upper surface of the sleeve 40 is positioned just .slightly higher than the flat support surface 38 of the pin. Afier the sleeve is so positioned the mass B is easily positioned upon .the upper end of the sleeve 40. The weight is then positioned to engage the top of the mass B, as clearly shown in FIG. 3. Finally, the sleeve is carefully moved in a downward direction to the position illustrated in FIG. 1 where it is spaced from the support surface 38 of the pin 36. Thus, the second massB is solely supported by the pin 36 between the support surface thereof and the weight 42.

The center of the mass B is recorded in the horizontal plane of the paper 54 by the plumb bob 58. This point may be identified as point B,, as shown in FIG. 4. It is important that the center of mass B before collision be slightly offset with respect to the line A-A. The mass A is positioned and travels along the track 12 in the same manner as described above. In the course of its travel, the mass A collides with the mass B and the points that these masses strike the paper 54 are recorded on the paper by the carbon 56. Point B, is the point mass B strikes the paper 54 after collision and point A, is the point the mass A strikes the paper 54 after colliding with mass B. The distance between points B, and B, represents the relative horizontal velocity of mass B after collision. The point A, of mass A at the moment of collision with mass B is located on line B,-B', at a distance equal to the sum of the radii of masses A and B as measured from point B, along B,B', in the direction of line A-A. The distance between points A, and A, is the relative horizontal velocity of mass A after collision.

Next, the angle of separation 0 between the relative horizontal velocities of the masses after collision is recorded. Also, for purposes explained below, the average direction of mass A before collision with respect to the direction of mass B after collision is recorded as angle The relative horizontal momentum of each mass after collision is computed by multiplying the relative horizontal velocity after collision by the mass of the sphere. Next, the momentum vector P of mass B is drawn, as shown in FIG. 5. This value is the relative horizontal momentum of mass B after collision. From the end of this vector the momentum vector P A of mass A is drawn so that the angle formed by P and P is the supplement of 0, the angle of separation of the relative horizontal velocity of the masses after collision. The vector P connecting the origin 0 and vector P is the total relative horizontal momentum after impact. This vector is measured and recorded. Also, the angle (1), of FIG. 5 is measured which gives the direction of the total relative horizontal momentum after collision with reference to the direction of mass B. This angle should closely compare with the angle Finally, the average relative horizontal momentum of mass A before impact is compared with the average total relative horizontal momentum after impact. If Newtons third law of motion is valid for collisions, momentum should be conserved and the average total relative horizontal momentum of the two masses following impact should compare favorably with the average relative horizontal momentum of mass A before impact. The average relative horizontal momentum of mass A before impact is computed by multiplying the mass of sphere A by the average relative horizontal velocity of mass A before impact.

The following data table is representative of the data obis rsqhyyti i ins t sarrmm .0 Q? h P in t m- Angle 952, direction the Angle or, total relative average direchorizontal Relative horizontal Relative horizontal Angle 0, tion of Mass A momentum Diflerenoe velocity after collision momentum alter collision Total horizontal Roll angle oi before the collialter the colliin angles momentum sequence separation sion slon r and 2 Mess A Mass B Mess A Mass B after collision Relative horizontal velocity of Mess A before impact, 73.61 (Average oi 6 runs).

Mass 01 Sphere A, 16.358 grams; Mass 018 here B, 21.376 grams.

Average relative horizontal momentum 0 Mass A before impact, 1204.

Average total relative horizontal momentum after impact, 1203.

Percent diflerence in momentum with reierence to the momentum of Mess A before impact, 0.1%.

stantially horizontal portion, a second substantially spherical, mass, and supporting means for the second mass connected.

for shifting movement into and out of alignment with the substantially horizontal portion of the path of travel of the first mass, the supporting means including a vertically disposed support pin with a flat support surface having an area substan-- tially less than the diametral cross-sectional area of the second mass, a sleeve snugly surrounding the support pin mounted for sliding movement relative to the pin, and a weight spaced from,

and directly above the flat support surface of the support pin for engaging the top of the second mass when the second mass is positioned upon the support surface of the pin, the sleeve having an upper position where the upper surface thereof and the support surface of the pin are close to one another for locating the second mass on the support surface and a lower position spaced from the support surface of the pin whereby the second mass is supported by the pin between the support surface thereof and the weight.

2. Apparatus as in claim 1 wherein the diametral cross-sec tional area of the second mass is at least one hundred times greater than the area of the flat support surface of the pin.

3. Apparatus as in claim 1 including a hinge assembly connected between the weight and an upright portion on the support means.

4. Apparatus as in claim 3 including an adjustable connection between the upright portion and the support means whereby the weight may be adjusted in the horizontal and vertical directions.

5. Apparatus as in claim 1 wherein the means for moving the first mass includes a track having a downwardly inclined first portion with a substantially horizontal second portion at the lower end of the inclined portion.

6. Apparatus as in claim 1 wherein the vertically disposed support pin is adjustably connected to the supporting means for adjustment in the vertical direction whereby the support pin may be located so as to osition the center of the second mass in the same honzonta plane as the center of the first the lower end of the inclined portion, and wherein the weight is hingedly connected to the supporting means for movement toward and away from the support pin, the hinge connection for the weight being located on the side of the support pin directly opposite the horizontal second portion of the track.

9. Apparatus as in claim 6 wherein the vertically disposed support pin is adjustably connected to the supporting means for adjustment in the vertical direction whereby the support pin may be located so as to position the center of the second mass in the same horizontal plane as the center of the first mass when the second mass is supported by the pin and the first mass is traveling along the horizontal portion of its path of travel. 

1. Apparatus for demonstrating Newton''s third law of motion comprising a first substantially spherical mass, means for moving the first mass along a path of travel that includes a substantially horizontal portion, a second substantially spherical mass, and supporting means for the second mass connected for shifting movement into and out of alignment with the substantially horizontal portion of the path of travel of the first mass, the supporting means including a vertically disposed support pin with a flat support surface having an area substantially less than the diametral cross-sectional area of the second mass, a sleeve snugly surrounding the support pin mounted for sliding movement relative to the pin, and a weight spaced from and directly above the flat support surface of the support pin for engaging the top of the second mass when the second mass is positioned upon the support surface of the pin, the sleeve having an upper position where the upper surface thereof and the support surFace of the pin are close to one another for locating the second mass on the support surface and a lower position spaced from the support surface of the pin whereby the second mass is supported by the pin between the support surface thereof and the weight.
 2. Apparatus as in claim 1 wherein the diametral cross-sectional area of the second mass is at least one hundred times greater than the area of the flat support surface of the pin.
 3. Apparatus as in claim 1 including a hinge assembly connected between the weight and an upright portion on the support means.
 4. Apparatus as in claim 3 including an adjustable connection between the upright portion and the support means whereby the weight may be adjusted in the horizontal and vertical directions.
 5. Apparatus as in claim 1 wherein the means for moving the first mass includes a track having a downwardly inclined first portion with a substantially horizontal second portion at the lower end of the inclined portion.
 6. Apparatus as in claim 1 wherein the vertically disposed support pin is adjustably connected to the supporting means for adjustment in the vertical direction whereby the support pin may be located so as to position the center of the second mass in the same horizontal plane as the center of the first mass when the second mass is supported by the pin and the first mass is traveling along the horizontal portion of its path of travel.
 7. Apparatus as in claim 1 including a plumb bob secured to the lower end of the vertically disposed support pin for locating the center of the second mass in the same horizontal plane as the tip of the plumb bob.
 8. Apparatus as in claim 1 wherein the means for moving the first mass includes a track having a downwardly inclined first portion and a substantially horizontal second portion at the lower end of the inclined portion, and wherein the weight is hingedly connected to the supporting means for movement toward and away from the support pin, the hinge connection for the weight being located on the side of the support pin directly opposite the horizontal second portion of the track.
 9. Apparatus as in claim 6 wherein the vertically disposed support pin is adjustably connected to the supporting means for adjustment in the vertical direction whereby the support pin may be located so as to position the center of the second mass in the same horizontal plane as the center of the first mass when the second mass is supported by the pin and the first mass is traveling along the horizontal portion of its path of travel. 